Context: Hormonal male contraceptive regimens effectively and reversibly suppress sperm production but there are few large-scale efficacy studies.

Objective: The safety, contraceptive efficacy, reversibility and feasibility of injectable testosterone undecanoate (TU) in tea seed oil as a hormonal male contraceptive was assessed.

Design: This was a mutilcenter, phase III, contraceptive efficacy clinical trial.

Participants: 1045 healthy fertile Chinese men were recruited throughout China into the study.

Intervention(s): Monthly injections of 500 mg TU, administered for 30 months. A definition of severe oligozoospermia ( 1 x 10⁶/ml) was used as a criterion of spermatogenic suppression and used as the threshold for entering the contraceptive efficacy phase.

Main Outcome Measure(s): The primary outcome is pregnancy rate in the partner. Other outcomes include: semen parameters, testis volumes, reproductive hormone levels, and safety laboratory tests.

Results: 43 participants (4.8%) did not achieve azoospermia or severe oligozoospermia within the 6-month suppression phase. 855 participants entered into the efficacy phase and 733 participants completed monthly TU treatment and follow-up. There were 9 pregnancies in 1554.1 person-years of exposure in the 24-month efficacy phase for a cumulative contraceptive failure rate of 1.1 per 100 men. The combined method failure rate was 6.1%, comprising 4.8% with inadequate suppression and 1.3% with post-suppression sperm rebound. No serious adverse events were reported. Spermatogenesis returned to the normal fertile reference range in all but two participants.

Conclusions: Monthly injection of 500 mg TU provides safe, effective, reversible and reliable contraception in a high proportion of healthy fertile Chinese men.